1. Field of the Invention
The present invention relates to an oxide superconductor and a method for producing the same.
2. Description of the Related Art
High critical-current oxide superconductor materials that have been developed in recent years are expected to be usefully applied, for example, to fusion reactors, magnetically levitated trains, particle accelerators, magnetic resonance imaging apparatuses (MRI) and microwave filters, and some of these technologies have been already used in practical fields.
Major oxide superconductors are bismuth-based, yttrium-based and thallium-based superconductors. Of these materials, yttrium-based superconductors have been attracting much attention as almost practically applicable materials since they exhibit the highest superconducting properties in a magnetic field at a liquid nitrogen temperature and can be used for magnetic levitated trains by cooling with liquid nitrogen.
The yttrium-based superconductor is expressed by a formula YBa2Cu3O7-x, and has a perovskite structure. Compounds in which yttrium is replaced with a lanthanoid group rare earth element, and mixtures thereof have been also known to exhibit superconducting properties. Examples of the method of producing these superconducting materials so far used include pulsed laser deposition (PLD), liquid phase epitaxy (LPE), electron beam. (EB) processing, and metalorganic deposition (MOD).
The methods of producing the superconducting material are roughly classified into an in situ process and an ex situ process. The in situ process performs deposition of metals essential for producing the superconductor and formation of a superconductor by oxidation the metals at once. The ex situ process independently performs deposition of metals essential for forming the superconductor and heat treatment for forming the superconductor. Accordingly, a precursor (calcined film) is formed in the ex situ process.
The in situ processes were paid attention as the production methods of the superconductor in the early stage. This is because the in situ processes were expected to make cost lowered due to its small number of production steps. However, production conditions are hard to be controlled in the processes since all the deposition conditions must be controlled at once, which has been revealed that good superconductors can hardly be obtained. On the other hand, although the ex situ process was afraid to increase the production cost, it has been possible to substantially reduce the production cost by developing a non-vacuum method such as an MOD method and a TFA-MOD method to be described below. It is an advantage of the ex situ process over the in situ process to divide the heat treatment into two steps in order to facilitate control of heat treatment.
Examples of the ex situ process include the EB processing (P. M. Mankiewich et al., Appl. Phys. Lett. 51, 1987, 1753-1755), MOD method, and TFA-MOD method (T. Araki and I. Hirabayashi, Supercond. Sci. Techol. 16, 2003, R71-R94).
In the EB processing, a precursor containing metals essential for the superconductor is deposited by electron beam, and a Y-based superconductor is then produced by applying a heat treatment (firing). A superconducting layer is supposed to be developed in the firing process via a quasi-liquid phase network in the presence of fluorine. Since no carbon is used in this method, there is no residual carbon at all in the superconductor obtained, and superconducting properties are not largely impaired. However, the EB processing involves a problem of high production cost.
The MOD method is a method that has been investigated in another field and is used in the production of the superconductor. While much effort has been made in order to reduce harmful residual carbon in the production of the Y-based superconductor by the MOD method, there is no effective method for reducing the residual carbon. Since organic substances in the precursor are decomposed by calcining in this method, the film obtained is also called a calcined film. The calcined film contains metal oxides and residual carbon and does not contain fluorine at all. The fired film also contains the metal oxides and residual carbon.
The TFA-MOD method derived from the MOD method will be finally described. A carbon expulsion scheme works during calcining by using trifluoroacetic acid (TFA) as a fluorine-containing compound in this method, and a calcined film from which most of carbon harmful for superconductor is expelled is readily obtained. It has been known that a highly oriented texture in an atomic level is formed with good reproducibility during the firing process, by forming a quasi-liquid phase network by the action of fluorine and by a chemical equilibrium reaction. Since no vacuum process is used at all from deposition to calcining and firing to enable the production cost to be reduced, research of this method has been spread worldwide. A wire material that enables 70 A of current with a wire having a length of 100 m can be produced with good reproducibility today. Consequently, the TFA-MOD method is a major process in the production of the yttrium-based superconductor.
However, while the TFA-MOD method has a large advantage that a superconductor exhibiting excellent superconducting properties may be produced with a low production cost, it is disadvantageously difficult to increase the thickness of the film. This is because reduction in the volume of the film is as large as 87% from a gel film to a final superconducting film in this process and a stress (drying stress) in the direction parallel to the substrate surface may be applied to the film during reduction in the volume, cracks are produced in the film when the film has a certain thickness (critical thickness) or more even by applying a moderate heat treatment. While a high-purity solution from which impurities are reduced as much as possible is usually used for obtaining a superconductor having excellent superconducting properties, the critical thickness in this case is about 300 nm. For example, when a superconductor with a thickness of 350 nm is formed on a substrate with a diameter of 2 inches, it has been confirmed that cracks with a width of 0.1 mm or more and a length of 1 mm or more that may be readily recognized by visual observation are formed.
Increasing the thickness of the superconductor will be described here. In the usual MOD method, the thickness of the film is increased by repeating the process of coating to form a gel film followed by calcining. The reason why such a process can be used is that the calcining is completed in a quite short period of time. However, it has been known that fatal degradation in superconducting properties is caused due to an increased amount of residual carbon when the thickness of the superconductor is increased by repeated coatings in the MOD method. On the other hand, in the TFA-MOD method, a gradually increasing time sufficient for breaking covalent bonds is necessary in the calcining step for decomposing organic substances by calcining while the organic substance is prevented from being burnt, and the longest time is spent in the entire process for calcining. Accordingly, a quite long time of heat treatment is necessary for increasing the thickness of the superconductor when coating is repeated in the TFA-MOD method. Homogeneity of the superconducting film is lost due to local crystallization when the film experiences many times of heat hysteresis, and the quality of the film is gradually degraded. In addition, it has been known that superconducting properties are degraded by repeated coatings since an oxide layer is formed at the boundary between the lower layer and upper layer formed by repeated coatings. In particular, increasing the thickness by repeated coatings causes a critical effect in the application of the superconductor for a microwave filter. Usually, a superconductor with a thickness of 400 nm or more is assumed to be necessary for the microwave filter, and a thick superconductor that maintains excellent superconducting properties is necessary particularly in a signal transmission side. However, since intermediate oxide layers are formed by repeated coatings as described above, adjustment of filter characteristics becomes quite difficult due to the oxide as a cause of loss to make it difficult to produce a sharp-cut filter. Accordingly, it is important to obtain a thick superconductor that exhibits excellent superconducting properties by single coating in the TFA-MOD method, in order to prevent superconducting properties from being degraded by long term heat treatment and by formation of oxide layers.
Next, a method for thickening the superconductor by single coating will be described below. In the usual MOD method, an organic compound having a longer chain is added to a metalorganic compound containing essential metals for forming the superconductor. In this case, the film is prevented for cracks from being produced by taking advantage of the fact that the added organic compound is not decomposed at a low temperature for decomposing the metalorganic compound that contains the essential metals. The added organic compound is decomposed thereafter at a higher temperature. The film thickness may be practically increased by using such a method. However, residual carbon originating from the added organic compound is a problem in the MOD method. On the other hand, it has been found in recent years that, when the same method is used in the TFA-MOD method, residual fluorine is problematic rather than residual carbon by virtue of the action of the carbon expulsion scheme. While a certain extent of fluorine is necessary in the TFA-MOD method for forming a quasi-liquid phase network in the firing process, the content of residual fluorine accounts for about 10 to 20 times when the same organic compounds as used in the MOD method are added as compared with the case when no such organic compound is added. It has been also anticipated that fluorine may be eliminated by keeping the firing condition for a long term since fluorine is eliminated when the quasi-liquid phase network is formed. However, it has been found that a minute amount of a texture of a Y-based superconductor in which fluorine compounds are mixed is formed by keeping the superconductor under the firing condition for a long period of time. The fluorine compound is recrystallized into barium fluoride during cooling, which leads to disturbance of crystal orientation. Actually, a superconductor has superconducting properties 1/10 or less of the original level when just a minute amount of BaF2 is detected by XRD measurement of the superconductor. Further, when the organic compound is added, the amount of residual fluorine increases by increasing the thickness of the film formed by single coating. This may be believed that fluorine existing at the bottom of the coating film is not eliminated.
Rupich et al have proposed a method for preventing the fluorine content in the oxide superconductor from increasing by using Cu carboxylate containing less fluorine in place of Cu trifluoroacetate without adding any fluorine-containing organic compounds (WO 2002/035615). Cu carboxylate used in this method contains, for example, chlorine, bromine or hydrogen in place of fluorine. However, since the solution used by Rupich et al is not a high-purity solution prepared by a Solvent-Intro-Gel method, it may be supposed that a certain amount of acetate salt remains already, which possibly increases the amount of residual fluorine.
While J. A. Smith et al have reported a superconductor having a thickness of as large as 1,000 nm, superconducting properties are not so high and the details of the production method are indefinite.